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getline
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getline can be used to read input from the current file or from another
file or from a pipe.

--------------------------------------
Form                What gets set
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getline             $0, NF,   NR, FNR
getline var         var,      NR, FNR
getline < file      $0, NF
getline var < file  var
cmd | getline       $0, NF
cmd | getline var   var

getline returns
	1 ... success
	0 ... EOF
	-1 ... failure (file not found, e.g.)

I. Reading from input file

Say you want to join two consecutive lines in the input file, "a".
$ cat a
A
B
C
 
You may naively try
$ awk '{getline x; print $0, x}' a
A B
C B

The reason is that you did not check the status of getline.
So you need to check the status of getline first.

$ awk '{if(getline x) print $0,x; else print $0;}' a
A B
C

II. Reading from a pipe.

Say you want awk to count the number of lines in a file "a" (not the one
you are working but some other file)..


$ awk 'BEGIN{"wc a" | getline x; split(x,b); print "num lines: "b[2]}'
num lines: 3

Of course if you want this program to be robust then you should check for
the return status of getline.